The present invention relates to a phase controllable optical delay interferometer, an optical differential phase shift keying (DPSK) demodulator using the same, and a demodulating method therefore. More particularly, the invention relates to an optical delay interferometer that couples delayed optical signals having different phases using a fiber grating such as an FBG (fiber Bragg grating) or an LPG (long period fiber grating) and a phase controller, an optical DPSK demodulator using the same, and a demodulating method therefore.
An ODI (optical delay interferometer) has been developed for a variety of applications including a multi-wavelength laser, an optical sensor system, and a demodulator for an optical differential phase shift keyed (hereinafter, referred to as optical DPSK) signal. In general, the ODI has been realized by an imbalanced MZI (Mach-Zehnder interferometer) with one bit time delay in one arm or an AMI (Asymmetric Michelson Interferometer). However, the MZI is intrinsically sensitive to environmental perturbations such as temperature, or acoustic interference, thus demanding an additional circuit for compensating them, and has a difficulty in precisely implementing the delayed amount. Further, the AMI is sensitive to the environmental perturbations and has a power loss of 3 dB.
As widely known, the modulation method for the optical communication system uses an intensity modulation method that has a simple structure, instead of a phase shifting method, which is different from the mobile communication using a wireless network. The phase shift keying method for optical communication is less sensitive to the nonlinearities and the dispersion. Accordingly, the phase shift keying method is suitable to transmit high speed signal through an optical fiber due to its inherently characteristic. However, it is difficult to maintain the phase of the optical signal while passing through the optical fiber, and the receiving device for detecting the phase shift is very complicated. Therefore, there are some problems to use the phase shift keying method.
However, the optical DPSK has recently attracted much attention as a suitable modulation format for long-haul high-speed transmission systems since it also has high tolerance to fiber nonlinearities and polarization mode dispersion during transmission. One of main components of the optical communication system using the above optical DPSK method is a demodulator disposed prior to the optical detector that converts the optical DPSK signal into an intensity signal.
The demodulator for an optical DPSK signal delays half an incident optical signal with one bit time delay, and converts an input DPSK signal into an intensity modulated signal on the basis of the phase difference between a delayed signal and non-delayed signal to output a converted signal. Therefore, in order to convert the optical DPSK signal into the intensity modulated signal without any distortion, additional phase shift other than an inherent phase difference of 0° or 180° (π) between two optical DPSK signals that propagate through the demodulator, that is, delayed signal and non-delayed signal should be not occurred. Further, the environmental fluctuations such as vibration or temperature changing should not influence the phase difference between the two optical DPSK signals.
The demodulator for optical DPSK signal according to the related art mainly uses the Mach-Zehnder interferometer having one-bit delay line. This demodulator includes optical elements for combining and dividing the light intensity into 3 dB, and one bit delayed light path and non-delayed light path that are displaced between the two optical elements.
However, when using the above demodulator, the delayed optical signal and the non-delayed optical signal pass through different optical paths, and the optical paths individually response to the external perturbations such as vibration or temperature changing. Therefore, it is difficult to precisely make the phase difference of the demodulator be 0° (when the phase difference is 0°, the demodulated signal has an inverted shape of the original signal. The original signal can be easily returned by the inverse detection of the receiver.) or 180° (π). As a result, it is difficult to perfectly convert the optical DPSK signal into an intensity modulated signal. In detail, since the demodulator according to the related art is sensitive to the external perturbations, the demodulator using the Mach-Zehnder interferometer requires to provide an additional circuit for compensating the external perturbations, which makes the system be complex. Further, since the optical signals pass through different optical paths, the optical signals are sensitive to the polarization due to the different refractive indices of the different optical paths. Further, since the one-bit delay line is implemented by the length difference between the two optical paths, the delay line may have the minute length error, which decreases the yield.
In order to solve the problem of the insensitivity to the external perturbations, another demodulating method is suggested to convert the DPSK signal into a polarization shift keying signal by using one optical high birefringence fiber and then re-convert the polarization shift keying signal into an intensity modulated signal using a polarizer. According to the demodulating method, one bit time delay is implemented by a time delay due to the birefringence. This demodulating method is not sensitive to the external perturbations because of using a single optical fiber. However, according to the above demodulating method, since the polarization axis of the input signal is always set to 45° with respect to optical high birefringence fiber, it is difficult to use the method in a common system that has a random polarization axis. Further, since the phase of the delayed optical signal is not changed, it is difficult to demodulate the different optical DPSK signal, and compensate the lowered performance of the system due to the change in the center wavelength of the input optical signal.